Semiconductor switching devices such as IGBTs (Insulated Gate Bipolar Transistors) will be thermally damaged if they are heated to excessive temperature due to high current operation. To prevent this, a temperature detector is used to detect the temperature of the chip.
FIG. 10 is a diagram showing the configuration of a conventional temperature detector (see, e.g., Japanese Laid-Open Patent Publication No. 2000-307403). Referring to the figure, a temperature sensing diode 13 is formed on the same chip 12 as a semiconductor switching device 11. A constant current circuit 14 supplies a constant current to the anode of the temperature sensing diode 13, and the anode potential of the temperature sensing diode 13 is used as a temperature signal indicating the temperature state of the semiconductor switching device 11. In this case, the circuit (not shown) that receives the temperature signal is designed to have a high input impedance, as is well known in the art. The forward voltage (Vf) of the temperature sensing diode 13 has a negative temperature coefficient (when the forward current is maintained constant), as shown in FIG. 11.
It should be noted that during the switching operation of the semiconductor switching device 11, negative noise is superimposed on the anode potential of the temperature sensing diode 13 due to wire inductance, loops, etc., resulting in a reduction in the anode potential. To prevent this, a CR filter 15 is inserted between the anode of the temperature sensing diode 13 and the output terminal to shape the temperature signal.
However, as the switching frequency increases, the potential of the temperature signal from the temperature sensing diode 13 gradually decreases, as shown in FIG. 12. Therefore, a conventional temperature detector may output an erroneous output signal even if the temperature signal from the temperature sensing diode 13 is shaped by the above filter, since the portions of the temperature signal affected by the negative noise may have a lower potential level than a predetermined threshold level.